Conventionally, electrophotographic image forming apparatuses such as copiers, printers, and facsimile machines have been known. Such electrophotographic image forming apparatuses are arranged such that (i) an electrostatic latent image is formed on the surface of a photoreceptor (toner image bearing member), (ii) the electrostatic latent image is supplied with toner from a developing device so as to be developed, and (iii) a toner image formed by such development is transferred onto a sheet such as a paper and is then fixed thereon.
In recent years, an image forming apparatus capable of forming high-quality color images frequently uses a two-component developer (hereinafter also referred to simply as “developer”), which has superior stability in charging of toner. The developer includes toner and carrier. Stirring the toner and the carrier in the developing device causes friction therebetween. This friction gives proper electrical charge to the toner. The electrically charged toner is supplied onto the surface of a developer bearing member (developing roller). The toner on the developer bearing member is moved by electrostatic attraction onto an electrostatic latent image formed on an image bearing member (photoreceptor). This consequently forms a toner image on the photoreceptor.
Recently, there has been a demand for a high-speed, downsized image forming apparatus. This has in turn necessitated rapid and accurate supply of a developer onto a toner image bearing member. In view of this, Patent Document 1 below discloses an image forming apparatus that employs a developing device of a circulation type for rapid and sufficient electrical charging of a developer and also for rapid transport of the developer. As shown in Patent Document 1, the developing device of the circulation type includes: a developer carrying path through which the developer is circulatively carried; and a developer carrying member by which the developer is simultaneously stirred in and carried through the developer carrying path. The developer carrying member is a screw having (i) a wing body in the form of a feed screw and (ii) a screen member like a mesh.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 63081/1998 (Tokukaihei 10-63081; published on Mar. 6, 1998)
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 89550/2000 (Tokukai 2000-89550; published on Mar. 31, 2000)
The above developing device of the circulation type is arranged such that, when the toner density of the developer in the developing device reaches below a predetermined value, toner is supplied to the developer carrying path from a toner hopper, a toner bottle or the like.
Unfortunately, according to the conventional developing devices of the circulation type, such toner supplied from the toner hopper to the developer carrying path would be circulatively carried in the form of aggregates in some cases, without being sufficiently mixed with the developer preexisting in the developing device already. In other words, the toner separated from the carrier would be circulatively carried. This disadvantageously causes insufficiently charged toner (toner having a small amount of charge) to be supplied onto the developer bearing member.
The supply of such insufficiently charged toner onto the developer bearing member more likely leads to a problem that the toner scatters off the developer bearing member. This makes the interior of the image forming apparatus dirty and/or stains a formed image. In addition, the supply of insufficiently charged toner onto the developer bearing member is also associated with another problem that electrostatic force for holding the toner on the surface of the carrier is consequently reduced. As a result, the toner is more likely adhered to an area where no image is to be formed. This causes photographic fog.